The present invention relates to interfaces for data transmission systems, of the type which comprise a plurality of devices linked by a highway via the interfaces. Devices include data acceptors, which are primarily addressed as listeners, although they may be addressed as talkers in poll and other command operations; data sources, which are primarily addressed as talkers but are also addressable as listeners during command operations; devices which are both sources and acceptors; and controllers which act as talkers and listeners in establishing the operational status of the overall system. A controller is generally also a data source and a data acceptor, and may be a device generally referred to as a data processor or a digital computer.
In operation a controller sets up a status in which one device is a talker and one or more devices are listeners. Data is then transferred from the talker to the listener(s). A system may include more than one controller but only one such device can exercise control at a time.
The invention is to be understood against the state of the art as represented for example by various draft standards being issued by Working Group 3 on Programmable Measuring Apparatus of Technical Committee No. 66 of the International Electrotechnical Commission, in particular the draft identified as 66/WG3 (Secretary) 7 of March 1974.
An interface controls communication between two ports. The term port is used herein to refer collectively to a set of terminals including a plurality of data terminals for bytes of data and a plurality of handshake terminals which are employed in exchanging handshake signals, for example in accordance with the aforementioned draft or in accordance with the invention described in the above-referenced copending Patent Application.